Method for induction lamination of electrically conductive fiber reinforced composite materials

ABSTRACT

A method and apparatus for forming laminate composite structures. At least two laminae, each containing electrically conductive reinforcing fibers, are placed upon each other in contacting relationship to form a generally layered structure. The layered structure may be subjected to heat to conductively transfer heat through the layered structure and thereby improve the surface contact between two laminae. The layered structure is volumetrically heated by inductively transferring energy to the electrically conductive reinforcing fibers. The heated, layered structure is consolidated, such as by applying pressure and reducing the temperature of the layered structure. The consolidated structure is then quenched by rapidly cooling the consolidated structure in a directionally controlled manner about a midplane thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/324,966, filed Sep. 25, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the formation of compositestructures and, more particularly, to the formation of substantiallyvoid-free laminate structures using inductive energy.

2. State of the Art

Composite articles are well known to provide advantages in diverseapplications. In some applications, the advantages of composite articlesover metal, ceramic or other materials include weight reduction and theability to integrate several otherwise individual parts into a singlestructure. Composite articles may conventionally include a reinforcementmaterial in a polymer-based resin, also known as the matrix material,such as thermoplastic or thermoset resin. The reinforcement materialsmay include, for example, chopped or continuous fibers disposed eitherrandomly or in ordered fashion within the polymer matrix material.Composite materials are conventionally formed by configuring thereinforced matrix material into a desired form or structure, which mayinclude heating the article to place the matrix material in a moldablecondition followed by resolidification in the case of a thermoplastic,and curing of the matrix material in the case of thermoset resins.

Conventionally, composite structures are often formed as laminates,meaning that laminae, or multiple lamina, are layered on top of eachother and bonded together by heating, thereby effecting melting inthermoplastics and effecting cross-linking between the multiple layersfor thermoset resins. Additionally, a consolidation process isconventionally carried out on laminate structures to increase adhesionand reduce voids between the laminae. Such consolidation isconventionally carried out by processes such as vacuum debulk or throughuse of an autoclave.

One limitation associated with manufacturing composite articles orstructures is the rate at which such articles may be processed,especially for composite articles exhibiting large cross-sectionalareas. For example, existing techniques such as autoclave, pultrusionand double belt press techniques are limited at the rate by whichthermal equilibrium can be achieved within the material duringprocessing.

Conventionally, heating of composite articles has been carried outthrough surface heating techniques. However, the thicker the compositestructure, the more difficult and time consuming it becomes to achievethe proper temperatures at or near the center of the structure. In orderto reduce the process or cycle times of producing a laminate compositestructure, the surface temperature may be increased in order to morequickly transfer thermal energy to the center of the compositestructure. Referring to FIG. 1, an exemplary graph 100 shows the need toincrease surface temperature of a composite structure in order toincrease the throughput, or the amount of material processed in a givenamount of time, in a conventional surface heating process. For example,the first plot 102 indicates that in order to maintain an exittemperature of 480° F. for a composite structure having a thermoplasticmatrix, the surface temperature of the composite structure duringprocessing must be increased approximately 1000° F. in order to realizea corresponding increase in the rate of throughput by approximately 19feet/second (ft/sec). Similarly, as seen in the second plot 104, tomaintain an exit temperature of 625° F. throughout the compositestructure, an increase in surface temperature of approximately 1200° F.is required to increase the throughput by approximately 19 ft/sec.

However, the allowable surface temperature of the composite laminatestructure is limited by its degradation temperature, which, in turn,limits the throughput or production rate. Thus, in using conventionalsurface heating techniques, the tradeoffs for improving production timesinclude an increase in both capital costs and labor (i.e., throughimplementation of parallel production lines) and/or the production of apotentially degraded and inferior product.

In an attempt to improve production times, inductive heating techniqueshave been implemented in the production of composite structures.Induction heating techniques conventionally take advantage of theinductive transfer of energy from an induction coil to a conductivemember either positioned adjacent a surface of the composite structureor disposed within the composite structure, such as between individuallaminae or in the matrix material of an individual lamina.

For example, U.S. Pat. No. 5,229,562 issued to Burnett et al. disclosesinductively heating a conductive member, such as a platen or a mandrel,which is positioned against a surface of the composite structure.However, such a method presents problems similar to those discussedabove since the use of platens or mandrels is simply another means ofsurface heating the composite structure.

Another inductive heating technique includes placing a conductivemember, such as a susceptor or a metal insert, into the composite andtransferring energy through the insert and into the body of thecomposite. Such a technique, which may generally be referred to hereinas volumetric heating, serves to bring the composite structure tothermal equilibrium much more efficiently. While volumetric heatingbrings the composite structure to thermal equilibrium much more quicklythan surface heating, the use of metal inserts or susceptors toaccomplish such may serve to mechanically weaken the resultingstructure.

Another inductive heating technique involves transferring inductiveenergy to electrically conductive reinforcing fibers placed within thematrix material of a composite structure. For example, U.S. Pat. No.4,871,412 issued to Felix et al., the disclosure of which isincorporated by reference herein, teaches the formation of local spotwelds and seam welds for lap joints between two composite structures byinductively transferring energy to carbon fibers disposed within the twostructures.

U.S. Pat. No. 5,357,085 issued to Sturman, Jr., the disclosure of whichis incorporated by reference herein, teaches the heating of a polymermatrix composite stand by passing the strand through a helical guidetube adjacent an inductive coil to transfer energy into carbon fibers ofthe composite strand.

U.S. Pat. No. 5,338,497 issued to Murray et al., the disclosure of whichis incorporated by reference herein, teaches forming a thick compositestructure by placing the composite material into a mold and inductivelytransferring energy into conductive elements, such as metal whiskers,disposed in the matrix material while the composite material is in themold.

However, while teaching volumetric heating of a composite structure, theabove-referenced processes fail to address the use of volumetric heatingin the high-volume production of laminate structures while retaininglaminate quality, including the prevention of internal voids or warping.While conventional consolidation techniques may be used to reduce voids,i.e., through the use of an autoclave or by subjecting the structure tovacuum debulk, such techniques are time consuming and also requireconsumable or disposable waste materials. Such waste materials mightinclude, for example, release films, bagging materials or nitrogen basedvolatiles used during pressurization.

In view of the shortcomings in the art, it would be advantageous toprovide an apparatus and a method for forming laminated compositestructures which allow for volumetric heating and prevent the subsequentgrowth of voids while also increasing throughput rates. It would befurther advantageous to provide an apparatus and method which allow forcontinuous production of a laminate composite structure without the needfor consumable waste materials conventionally used during consolidation.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method is provided forforming a composite laminate structure. The method includes providing atleast two laminae containing electrically conductive reinforcing fibers,the two laminae being configured in a layered arrangement. The layeredarrangement is volumetrically heated by inductively transferring energyto the electrically conductive reinforcing fibers contained in the atleast two laminae. At least a portion of the layered arrangement iscooled while pressure is substantially simultaneously applied thereto.The layered arrangement is substantially symmetrically quenched about amidplane of the layered arrangement to reduce the temperature of theinterior portion of the layered arrangement and to prevent the growth ofinternal hot voids between the laminae and warping of the resultantstructure.

In accordance with another aspect of the invention, another method isprovided for forming a composite laminate structure. The method includesproviding at least two laminae containing electrically conductivereinforcing fibers, the two laminae being configured in a layeredarrangement. The layered arrangement is volumetrically heated byinductively transferring energy to the electrically conductivereinforcing fibers contained in the at least two laminae. At least aportion of the layered arrangement is cooled while pressure issubstantially simultaneously applied thereto. The layered arrangement isasymmetrically quenched relative to a midplane of the layeredarrangement to reduce the temperature of the interior portion of thelayered arrangement to form the layered arrangement into a desiredshape, or to provide the resulting structure with a preconfigured stressstate.

In accordance with yet another aspect of the present invention, afurther method of forming a composite laminate structure is provided.The method includes providing a first lamina of thermoplastic materialcontaining a first plurality of electrically conductive reinforcingfibers. At least one other lamina of thermoplastic material containinganother plurality of electrically conductive reinforcing fibers isplaced upon the first lamina of thermoplastic material to form a layeredarrangement. The surface of the layered arrangement is heated toincrease contact between the first lamina of thermoplastic material andthe at least one other lamina of thermoplastic material. The layeredarrangement is volumetrically heated and melted by inductivelytransferring energy to the electrically conductive reinforcing fibers.The melted layered arrangement is then consolidated, such as byapplication of pressure with a cooled roller. The consolidated layeredarrangement is quenched substantially symmetrically about a midplanethereof to reduce the internal temperature and to prevent the growth ofinternal voids and warping of the resultant structure.

In accordance with a further aspect of the present invention, yetanother method of forming a composite laminate structure is provided.The method includes providing a first lamina of thermoset materialcontaining a first plurality of electrically conductive reinforcingfibers. At least one other lamina of thermoset material containinganother plurality of electrically conductive reinforcing fibers isplaced upon the first lamina of thermoset material to form a layeredarrangement. The layered arrangement is volumetrically heated andpartially cured by inductively transferring energy to the electricallyconductive reinforcing fibers. The partially cured layered arrangementis then consolidated, such as by application of pressure with a cooledroller. The consolidated layered arrangement is quenched substantiallysymmetrically about a midplane thereof to reduce the internaltemperature and to prevent the growth of internal voids and warping ofthe resultant structure.

In accordance with another aspect of the invention, an apparatus forproducing a composite laminate structure is provided. The apparatusincludes a preheating zone configured and located to heat a surface of alayered structure. An induction coil is configured and located totransfer energy to a plurality of electrically conductive reinforcingfibers contained in the layered structure. A consolidation zone isconfigured and located to remove voids from the layered structuresubsequent to the transfer of inductive energy to the layered structure.A quenching zone is configured and located to rapidly cool the layeredstructure symmetrically about a midplane thereof subsequent to theconsolidation of the layered structure, thereby preventing the growth ofhot internal voids. A drive is configured and located to continuouslyconvey the layered structure through the preheating zone, past theinduction coil, through the consolidation zone, and through thequenching zone.

In accordance with another aspect of the invention, an apparatus forproducing a composite laminate structure is provided. The apparatusincludes a preheating zone configured and located to heat a surface of alayered structure. An induction coil is configured and located totransfer energy to a plurality of electrically conductive reinforcingfibers contained in the layered structure. A consolidation zone isconfigured and located to remove voids from the layered structuresubsequent to the transfer of inductive energy to the layered structure.A quenching zone is configured and located to rapidly cool the layeredstructure asymmetrically relative to a midplane thereof subsequent tothe consolidation of the layered structure to form the layered structurein a desired shape or to induce a desired stress state into thestructure. A drive is configured and located to continuously convey thelayered structure through the preheating zone, past the induction coil,through the consolidation zone, and through the quenching zone.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a graph showing the throughput of a composite structurerelative to the surface temperature of the composite structure based onprior art surface heating techniques.

FIG. 2 shows an apparatus for processing laminate composite structuresaccording to one embodiment of the present invention;

FIG. 3 is an enlarged view of a portion of the apparatus shown in FIG.2;

FIG. 4 is an isometric view of a fiber reinforced laminae;

FIG. 5 is a plan view of a layered structure with individual laminapeeled back to show certain aspects of the layered structure;

FIG. 6 is a partial cross-sectional view of an induction coil accordingto one aspect of the present invention;

FIG. 7 shows an apparatus for processing laminate composite structuresaccording to another embodiment of the present invention; and

FIG. 8 shows an apparatus for processing laminate composite structuresaccording to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, an apparatus 200 for producing laminated compositestructures 202 is shown. The apparatus may be generally referred toherein as a laminator. The laminator 200 may be described as havingvarious zones, with each zone providing a specified function as shall bedetailed below. The laminator 200 includes a drive 204 for conveying thecomposite structure 202 through the various zones thereof. The drive 204may include one or more drive rollers 206A, 206B at the inlet 208 and/oroutlet 210, respectively, of the laminator 200. Alternatively, the drivemay include other components such as a conveyor system. The drive 204 isdesirably configured to convey the composite structure 202 continuouslythrough the laminator 200 and is adjustable such that differentcomposite structures having, for example, different thicknesses may beprocessed at different throughput speeds. The drive 204 is alsoconfigured to provide adjustable tension in the reinforcing fibers 218(FIGS. 3 and 4) of the composite structure 202 as it is being conveyedthrough the laminator 200. The application of a desired amount oftension within the composite structure 202 reduces fiber waviness andimproves resultant mechanical and dimensional properties thereof. Theamount of tension provided by the drive 204 may be based, in part, onthe tensile strength and modulus of the reinforcing fibers 218.Additionally, if desired, the amount of tension provided by the drive204 may also provide a predetermined amount of “prestress” in theresultant structure.

The composite structure 202 initially begins as a layered structure 212or a layered arrangement of lamina 214. The layered structure 212 mayinclude various materials and exhibit various configurations as will beappreciated and understood by those of ordinary skill in the art. Forexample, the layered structure 212 may include a stack of prepreglayers, commingled fabrics or a mix of thermoplastic layers and wovenfabrics or 3-D preforms. The fabric in such structures may include, forexample, electrically conductive material such as carbon fibers. Inaddition to containing an electrically conductive material, the layeredstructure 212 may include nonelectrically conductive reinforcingmaterial.

Referring briefly to FIG. 3, an isometric cross-sectional view of anexemplary lamina 214 shows that the lamina 214 may be formed of a matrixmaterial 216 having reinforcing fibers 218 disposed therein. The matrixmaterial 216 may include a thermoplastic polymer such as, for example,polyetherimide (PEI) or polyetheretherketone (PEEK). Alternatively, thematrix material may include a thermoset polymer such as an epoxy. Thereinforcing fibers 218 may be formed of an electrically conductivematerial and may include, for example, carbon fibers. Such electricallyconductive reinforcing fibers 218 are sufficiently conductive so as togenerate heat when exposed to an alternating magnetic field.

An individual lamina 214 may have the reinforcing fibers 218 oriented ina unidirectional manner to provide the lamina 214 with certain desirablemechanical properties. In such a case, laminae 214 may be stacked orlayered such that the reinforcing fibers 218 of a given lamina 214 areangled with respect to the reinforcing fibers 218 of another lamina inthe layered structure 212. For example, referring to FIG. 4, a layeredstructure 212 is shown including four laminae 214A-214D with some of thelaminae “peeled” back to reveal the orientation of their respectivereinforcing fibers 218.

The example shown in FIG. 4 includes a first lamina 214A havingreinforcing fibers 218 unidirectionally oriented in what may be termedthe 0° direction. The 0° direction of the reinforcing fibers in lamina214A may also be the intended direction of travel of the compositestructure 202 as it is conveyed through the laminator 200. Thereinforcing fibers 218 of the second lamina 214B are unidirectionallyoriented at an angle, for example at 30°, relative to the first lamina214A. The reinforcing fibers 218 of the third lamina 214C areunidirectionally oriented in the 0° direction, or substantially parallelwith those of the first lamina 214A. The reinforcing fibers 218 of thefourth lamina 214D are unidirectionally oriented at an angle, such as at−30° relative to those of the first and third laminae 214A and 214C.Additional lamina 214 may be added with a similar repeating pattern. Ofcourse, other patterns are contemplated and may be incorporated into thelaminated composite structure in accordance with the present invention.

The arrangement of the layered structure 212 shown in FIG. 4 servesmultiple purposes. As noted above, the orientation of the reinforcingfibers 218 serves to determine the mechanical properties of theresulting composite structure. For example, the tensile modulus andstrength of the composite structure is affected by the relativeorientation of the reinforcing fibers 218. Additionally, the overlappingnature of the reinforcing fibers 218 according to the arrangement shownin FIG. 4 allows for an electrical circuit to be completed within therespective laminae 214 when the layered structure 212 is subjected tothe inductive flux generated by an induction apparatus 234 (FIG. 2). Alayered structure 212 having all the reinforcing fibers 218unidirectionally oriented in the same direction (i.e., no overlapping)will not become volumetrically heated by induction since a closedelectrical pathway cannot be formed with the reinforcing fibers 218.

Alternatively, instead of having conductive reinforcing fibers 218 inevery lamina 214 of the layered structure 212, at least one of thelamina 214 might include conductive reinforcing fibers 218 arranged inan overlapping arrangement, or a conductive fiber mesh might be disposedwithin an individual lamina 214 with adjacent lamina includingnonconductive reinforcing members. Thus, an internal lamina 214 with theconductive reinforcing members might be used in generating internalheat.

Referring back to FIG. 2, the layered structure 212 is first subjectedto a preheat zone 220 which includes surface heaters 222 for heating thesurface of the layered structure 212. By preheating the layeredstructure 212, surface contact between the laminae 214 is increased,which may aid in effective volumetric heating of the layered structure212, a subsequent step in the process. The surface heaters 222 shown inFIG. 2 include platens 224 having heating elements 226 formed therein.Such heating elements 226 may include, for example, electricalresistance heaters, or radiant heating coils.

The preheat zone 220 may also include one or more sensors fordetermining the temperature of the layered structure 212 or the surfaceheaters 222 or both. For example, an infrared sensor 228 may be utilizedto determine the surface temperature of the layered structure 212.Similarly, a thermocouple 230 or like device might be placed in theplatens 224 of the surface heaters 222 to monitor their temperature andperformance. Furthermore, the sensor 228 and thermocouple 230 might beincorporated as part of a control loop such as, for example, closed loopfeedback control of the surface heaters 222 for greater control of thetemperature thereof.

For example, it may be desirable to preheat the layered structure byapplication of heat at approximately 300° C. to improve surface contactbetween the lamina 214. Additionally, it may be desirable to maintainthe temperature of the platens 224 within a specified range, such as,for example, within (±) 2° C. of a desired temperature. In anotherembodiment, it may be desirable to apply heat at a temperature ofapproximately 375° C. and maintain the temperature within (±)approximately 3° C.

It is noted that the preheat zone 220 is more beneficial in processing alayered structure 212 having lamina 214 formed of a thermoplasticmatrix. Thus, if desired, the preheat zone 220 might be disabled duringthe processing of a layered structure 212 having lamina 214 formed of athermoset matrix.

After passing through the preheat zone 220 of the laminator 200, thelayered structure enters a volumetric heating zone 232. The volumetricheating zone 232 may include an indication apparatus 234 fortransferring energy to the electrically conductive reinforcing fibers218 (FIGS. 3 and 4) of the laminae 214 in the layered structure 212.

Referring to FIG. 5, an enlarged view of the preheat zone 220 andvolumetric heating zone 232 is shown. As described above, the surfaceheater 222 heats the surface of the layered structure 212 such that heatis transferred conductively therethrough as indicated by arrows 236.However, in the volumetric heating zone 232, the inductive coilstransfer energy to the electrically conductive reinforcing fibers 218such that heat is generated at multiple locations throughout the volumeof the layered structure 212 as indicated by arrows 238. Byvolumetrically heating the layered structure 212, a thermal equilibriumwithin the layered structure 212 is quickly and efficiently obtained.For example, by using inductive heating, the temperature of the layeredstructure 212 might be increased at a rate of approximately 100° C. persecond. Of course, such rates might be higher or lower with the upperlimit largely being defined by the characteristics of the matrixmaterial being processed. Thus, the internal lamina 214 may be heated tothe required temperature very quickly without overheating the surface ofthe layered structure 212 which might result in material degradation.

Referring to FIG. 6, an exemplary induction apparatus 234 is shown. Theinduction apparatus 234 includes an induction coil 242 which may be castin a nonconductive polymer 244. One or more ceramic platens 246 may bepositioned to isolate the induction coil from the high temperatures ofthe layered structure 212. Additionally, a coolant line 248 may beprovided to maintain the temperature of the induction apparatus 234 ator below a specified temperature. As will be understood and appreciatedby those of skill in the art, the induction coil 242 may be designed toimpart a desired inductive flux pattern. Thus, in conjunction with thepresent invention, the induction coil 242 may be designed to produce aspecific pattern which optimizes the transfer of energy to the layeredstructure 212 depending on the fibrous patterns formed therein.

For example, optimum transfer of energy to a layered structure 212exhibiting a pattern of reinforcing fibers 218 such as is shown in FIG.4 may require an induction coil 242 having a first design. On the otherhand, optimum transfer of energy to a layered structure 212 exhibiting apattern of reinforcing fibers 218 arranged at substantially 90° anglesfrom one lamina 214 to an adjacent lamina 214 may require an inductioncoil 242 having a different design.

The induction coil 242 is desirably adjustable with respect to bothfrequency and power. Thus, for example, the induction coil may beoperated at a frequency of 2.3 MHz when processing a layered structure212 formed of thermoplastic and exhibiting a thickness of approximately1 mm. However, the induction coil 242 may be operated at a differentfrequency when processing a layered structure 212 formed of a differentmaterial or which exhibits a different thickness. Likewise, the powerlevel of the induction coil might be adjusted to accommodate layeredstructures 212 of varied compositions and arrangements.

It is further noted that the induction coil 242, in conjunction with thelayered pattern of reinforcing fibers 218, is desirably configured so asto generate heat uniformly throughout the layered structure 212. Thisdesirably includes uniformity across the width of the layered structure212 as well as through the thickness of the layered structure.

Referring back to FIG. 2, the volumetric heating zone 232 efficientlyheats the laminae 214 of the layered structure 212 to a sufficienttemperature resulting in the melting thereof in the case of athermoplastic matrix, or the curing thereof (or more particularly, thepartial curing thereof) in the case of a thermoset matrix. By heatingthe layered structure 212, the laminae 214 bond with one another tocreate a more unified structure as will be appreciated by those ofordinary skill in the art.

The volumetric heating zone 232 may further include one or moretemperature sensors such as, for example, an infrared sensor 240 formonitoring the temperature of the layered structure and controlling theinduction apparatus 234 as is necessary. For example, given a specifiedmaterial composition and thickness, it may be desirable to raise thetemperature of the layered structure 212 to within 10° C. of apredetermined temperature.

A consolidation zone 250 follows the volumetric heating zone 232 and mayinclude, for example, one or more rollers 252 configured to applypressure to the layered structure 212 to further bond and to removevoids between the melted laminae 214. The rollers may be configured toapply pressure according to a specified pressure-time profile dependingon, for example, the type of material being processed, the temperatureof the layered structure 212 or the thickness of the layered structure212.

The rollers 252 are desirably maintained at a reduced temperaturerelative to the temperature of the layered structure 212. For example,the rollers 252 are desirably maintained at a temperature below theglass temperature (T_(g)) of the matrix material in the laminae 214.Thus, in one embodiment, the rollers 252 may be maintained at ambient orroom temperature for cooling the surface of the layered structure 212.One advantage of maintaining the rollers 252 at a reduced temperature isthe prevention of resin buildup on the rollers 252, which may occur dueto pressurized contact with the surface of the layered structure 212.The rollers 252 may be maintained at a specified temperature by, forexample, flowing a coolant through the interior of the rollers 252 or byother means as will be recognized by those of ordinary skill in the art.

While alternative pressure mechanisms such as platens may be used toeffect consolidation, the use of a roller 252 amplifies the appliedpressure due to the small area of contact between the roller 252 and thelayered structure 212. An exemplary roller 252 may be made of stainlesssteel and is configured to resist bending during application of apredetermined maximum amount of pressure to the layered structure 212,thereby ensuring a uniform application of pressure and a uniformthickness of the resultant composite structure 202. Additionally, it isdesirable that the rollers 252 be adjustable so as to accommodatelayered structures 212 of varied thicknesses as well as to vary theamount of pressure being applied to the layered structure 212. Thepressure applied through the roller 252 is used to prevent growth ofvoids in the composite structure and achieve the desired mechanicalproperties.

After passing through the consolidation zone 250, the layered structure212 passes through a quenching zone 260. The quenching zone 260 includesa cooling apparatus 262 for cooling the layered structure 212 in adirectionally controlled manner about a centerline, or a midplane 264,of the layered structure 212. By rapidly cooling the layered structure212, hot internal voids within the material are prevented from growingto unacceptable levels subsequent to the consolidation process.Additionally, the cooling of the layered structure 212 in adirectionally controlled manner may include, for example, cooling thelayered structure 212 substantially symmetrically about the midplane264, which prevents the layered structure 212 from warping and therebyproduces a resultant structure which exhibits less variance from onecomposite structure to another.

The cooling apparatus 262 may include one or more platens 266A, 266B(collectively referred to herein as “platens 266”) maintained at adesired temperature by flowing coolant through one or more passages 268formed in the platens 266. The platens 266 may also be configured toapply a predetermined amount of pressure, desirably a reduced amount ofpressure relative to the rollers 252 of the consolidation zone 250, tothe layered structure 212. The pressure applied by the platens 266,combined with the rapid cooling, serves to prevent growth of hotinternal voids in the interior of the layered structure 212 as theconsolidation zone 250 does not substantially cool the interior of thelayered structure 212. Additionally, the quenching zone reduces warpingin the resultant composite structure 202. The quenching zone 260 servesto reduce the temperature of the layered structure 212 about itsmidplane 264 below the glass transition temperature of a thermoplasticmatrix material when such material is being utilized.

In another embodiment, the quenching zone 260 may be configured suchthat the upper cooling platen 266A is maintained at a differenttemperature than that of the lower cooling platen 266B such that thecooling in a directionally controlled manner includes controlledasymmetric cooling of the layered structure 212. Controlled asymmetriccooling of the layered structure 212 allows for the layered structure tobe formed into a desired shape and/or allows for a predetermined stressstate to be imparted to the resultant structure. For example, bymaintaining a specified temperature differential between the upper andlower platens 266A and 266B, with the lower platen having the reducedtemperature, the resultant composite structure will exit the laminator200 curving downwards with a specified radius of curvature.Additionally, the temperature of the platens 266A and 266B may beindividually adjustable so as to better define the resultant shape ofthe composite structure 202.

Such asymmetric quenching provides an advantage of forming the layeredstructure 212 into a predefined shape during the lamination process,rather than having to preform subsequent operations to the layeredstructure 212 thereafter.

The quenching zone 260 may also include one or more sensing devices suchas, for example, an infrared sensor 270 for monitoring the temperatureof the layered structure 212 and/or a device such as a thermocouple 272for monitoring the temperature of the platen 266. The sensing devicesmay, as disclosed above, be incorporated as part of a control loop asmay be desired.

A substantially void-free (i.e., less than 1% void by volume) laminatedcomposite structure 202 exits the quenching zone 260 and the laminator200 having been processed at an increased rate of throughput relative toconventional methods including surface heating and vacuum debulk orautoclave consolidation processes.

A controller 274 may be operatively coupled with the laminator 200 toprovide automation of the process based on temperature, pressure andtime profiles for a given material at a given thickness. Thus, inoperation, an operator may feed the layered structure 212 into thelaminator 220 and the remaining process will be automated. Such controlsare known in the art and are not described in greater detail herein.Additionally, the controller 274 may be coupled with one or more of thevarious sensing devices (e.g., 228, 230, 240, 270, 272) to form variousfeedback loops in controlling the laminator 200.

It is further noted that the layered structure 212 processed by thelaminator 200 may be an individual component, such as a dimensionalsheet or panel, which may be processed in what is referred to as adiscontinuous feed mode, meaning that each individual dimensional sheetor panel must be individually fed through the machine. In such a case,for example, the individual sheets of panels may be fed into the inlet208 by a conveyor or by the front drive rollers 206A, which then feedsthe sheet through the laminator 200 to the rear drive rollers 206B. Infeeding an individual sheet in such a manner, it is noted that only halfa sheet is processed as it is fed through the laminator 200. Thus, thefirst half of the sheet (i.e., a dimensional layered structure 212) isnot initially processed as the laminator 200 does not begin to heat thematerial until the rear drive rollers 206B place tension in thereinforcing fibers 218 of the layered structure 212. Therefore, whenoperating in a discontinuous feed mode, the individual sheet will be fedthrough the laminator 200 a first time to process one half of the sheet,and subsequently fed through the laminator 200 a second time, orientedat 180° from the first time, to process the second half of the sheet.

Alternatively, the layered structure 212 may comprise a continuous rollof material (or continuous rolls of lamina 214) fed through thelaminator 200. In such a case, the process may be referred to as acontinuous feed mode. When operating the laminator in a continuous feedmode, the resulting composite structure 202 may be fed through a cuttingand trimming apparatus 290 which may be incorporated with the laminator200 or may be an independent apparatus.

Regardless of whether the laminator is operated in a continuous feedmode or a discontinuous feed mode, the lamination of the layeredstructure 212 (whether it be a dimensional sheet or a continuous feed ofthe layered structure 212) may be described as being a continuousprocess since the layered structure 212 may be continually fed from onestage or zone to another and since one process is being performed on oneportion of the layered structure while another process is simultaneouslybeing performed on another portion of the layered structure. Of course,in referring to the lamination as a continuous process, the term ismeant to take into account natural work stoppages, the feeding ofindividual sheets and so forth. Additionally, as discussed above herein,the continual nature of the process may be adjusted to accommodatevaried processing rates based on such factors as material selection andgeometries.

In another manner of describing the above process, a layered structure212 may be provided in the laminator 200 wherein various portions of thelayered structure experience different operations at substantially thesame time. For example, as seen in FIG. 2, a single layered structuremay have a first portion being preheated, a second portion beingvolumetrically heated; a third portion being consolidated, and a fourthportion being quenched, all substantially simultaneously. Of course,depending on such factors as the configuration of the laminator 200, thephysical size of the layered structure 212, whether the laminator isoperated in continuous or discontinuous feed mode, and the type ofmaterial being processed, the layered structure 212 may experience feweractivities at a given time.

Referring to FIG. 7, another embodiment of the laminator 200′ is shown.The laminator 200′ includes the same stages or zones as shown anddescribed with respect to FIG. 2. Thus, the laminator 200′ includes apreheat zone 220′, a volumetric heating zone 232, a consolidation zone250, and a quenching zone 260′. However, the preheat zone 220′ includesa plurality of heated rollers 280 in lieu of the platens or shoes 224 ofFIG. 2. Additionally, the quenching zone 260′ includes a plurality ofchilled rollers 282 in place of the platen or shoe 266 shown in FIG. 2.

Referring to FIG. 8, yet another embodiment of the laminator 200″ isshown. Again, the laminator 200″ is similar to the laminator 200depicted in FIG. 2; however, the preheat zone 220″ includes anoncontacting heat source 222′ which may include, for example, aninfrared heater or a forced gas heater 284.

EXAMPLE

As an example of the process and operation of the apparatus describedabove, a laminator similar to that described in FIG. 2 was constructedand tested with an 8 ply layered structure. The individual plies orlamina included a PEI matrix reinforced with carbon fiber and were 12inches by 36 inches in dimension. The laminator was configured toprocess the layered structure at a throughput rate of 5 feet per minute(fpm) in both discontinuous and continuous feed modes (denoted as“discont.” and “cont.” respectively in TABLE 1 below). The carbon fibersof the layered structure were oriented in the pattern as describedherein with respect to FIG. 4 above, i.e., [0/30/0/-30]_(s).

The resulting mechanical properties of the laminate composite structureproduced according to the above-described process were compared with asimilar laminate composite structure formed using a vacuum debulkprocess and are shown in TABLE 1 below.

TABLE 1 Longitudinal Longitudinal Transverse Longitudinal TensileTensile Tensile Tensile Cycle Time Process Strength (ksi) Modulus (msi)Strength (ksi) Modulus (msi) (seconds) Vacuum 191.7 ± 7.1 13.3 ± 0.516.3 ± 1.1 1.45 ± 0.04 300 Debulk Laminator @ 182.4 ± 2.8 13.6 ± 0.316.5 ± 0.3 1.5 ± 0.03 72 (discont.) 5 ft/min 36 (cont.)

As is shown in TABLE 1, the throughput rates (cycle times) when usingthe laminator were dramatically improved and the mechanical propertieswere substantially the same, and in most cases slightly improved, whencompared to the vacuum bulk process. Additionally, the consistency ofthe mechanical properties was improved over that of the vacuum debulkprocess, i.e., the variance was reduced.

It is noted that the cycle time for the laminator in discontinuous feedmode was twice that of the cycle time for the laminator in continuousfeed mode. This is a result of feeding the material through twice in thediscontinuous feed mode in order to process or laminate the entire sheetas has been described above.

The resulting structure exhibited a cross section with a majority of anyvoids being toward the edge or the surface of the structure and havingfewer voids in the center of the structure. The laminate quality metricsof the resulting structure included flatness of less than 1.5% andaverage void content of less than 1% as understood in the art.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the inventionincludes all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A method of forming a composite laminate structure, the methodcomprising: providing at least two laminae containing electricallyconductive reinforcing fibers, the at least two laminae being configuredin a layered arrangement; volumetrically heating the layered arrangementby inductively transferring energy to the electrically conductivereinforcing fibers of the at least two laminae; substantiallysimultaneously cooling and applying pressure to at least a portion of asurface of the layered arrangement; and quenching the layeredarrangement in a directionally controlled manner with respect to amidplane thereof including substantially symmetrically quenching thelayered arrangement with respect to the midplane thereof.
 2. The methodaccording to claim 1, further comprising surface heating the layeredarrangement prior to volumetrically heating the layered arrangement. 3.The method according to claim 2, wherein the surface heating includescontacting the at least a portion of the surface of the layeredarrangement with at least one heated platen.
 4. The method according toclaim 3, further comprising determining a temperature of the layeredarrangement during the surface heating of the layered structure.
 5. Themethod according to claim 4, further comprising maintaining atemperature of the at least one heated platen within approximately 2° C.of a predetermined temperature.
 6. The method according to claim 2,wherein the surface heating, the volumetrically heating, thesubstantially simultaneously cooling and applying pressure, and thequenching in a directionally controlled manner are carried out as acontinuous process.
 7. The method according to claim 1, furthercomprising determining a temperature of the layered arrangement whilethe layered arrangement is being volumetrically heated.
 8. The methodaccording to claim 7, further comprising maintaining the temperature ofthe layered arrangement within 10° C. of a predetermined temperature. 9.The method according to claim 1, wherein the volumetrically heating thelayered arrangement includes heating the layered arrangement at a rateof up to 100° C. per second.
 10. The method according to claim 1,wherein the providing at least two laminae containing electricallyconductive reinforcing fibers includes orienting the reinforcing fibersof one lamina of the at least two laminae at a specified angle relativeto the reinforcing fibers of at least one other lamina of the at leasttwo laminae.
 11. The method according to claim 1, wherein providing atleast two laminae includes providing a first lamina of thermoplasticmaterial and at least one other lamina of thermoplastic material, andwherein volumetrically heating the layered arrangement further includesmelting the first lamina and the at least one other lamina ofthermoplastic material.
 12. The method according to claim 1, whereinproviding at least two laminae includes providing a first lamina ofthermoset material and at least one other lamina of thermoset material,and wherein volumetrically heating the layered arrangement furtherincludes effecting partial curing of the layered arrangement.
 13. Themethod according to claim 1, wherein the volumetrically heating iscarried out on a first portion of the layered arrangement, thesubstantially simultaneously cooling and applying pressure is carriedout on a second portion of the layered arrangement and the quenching ina directionally controlled manner is carried out on a third portion ofthe layered arrangement in a substantially simultaneous manner.
 14. Amethod of forming a composite laminate structure, the method comprising:providing at least two laminae containing electrically conductivereinforcing fibers, the at least two laminae being configured in alayered arrangement; volumetrically heating the layered arrangement byinductively transferring energy to the electrically conductivereinforcing fibers of the at least two laminae; substantiallysimultaneously cooling and applying pressure to at least a portion of asurface of the layered arrangement; and quenching the layeredarrangement in a directionally controlled manner with respect to amidplane thereof including asymmetrically quenching the layeredarrangement with respect to the midplane thereof.
 15. The methodaccording to claim 14, further comprising surface heating the layeredarrangement prior to volumetrically heating the layered arrangement. 16.The method according to claim 15, wherein the surface heating includescontacting the at least a portion of the surface of the layeredarrangement with at least one heated platen.
 17. The method according toclaim 16, further comprising determining a temperature of the layeredarrangement during the surface heating of the layered structure.
 18. Themethod according to claim 17, further comprising maintaining atemperature of the at least one heated platen within approximately 2° C.of a predetermined temperature.
 19. The method according to claim 14,wherein the surface heating, the volumetrically heating, thesubstantially simultaneously cooling and applying pressure, and thequenching in a directionally controlled manner are carried out as acontinuous process.
 20. The method according to claim 14, furthercomprising determining a temperature of the layered arrangement whilethe layered arrangement is being volumetrically heated.
 21. The methodaccording to claim 20, further comprising maintaining the temperature ofthe layered arrangement within 10° C. of a predetermined temperature.22. The method according to claim 14, wherein the volumetrically heatingthe layered arrangement includes heating the layered arrangement at arate of up to 100° C. per second.
 23. The method according to claim 14,wherein the providing at least two laminae containing electricallyconductive reinforcing fibers includes orienting the reinforcing fibersof one lamina of the at least two laminae at a specified angle relativeto the reinforcing fibers of at least one other lamina of the at leasttwo laminae.
 24. The method according to claim 14, further comprisingforming the layered arrangement to a desired shape through theasymmetric quenching thereof.
 25. The method according to claim 14,wherein providing at least two laminae includes providing a first laminaof thermoplastic material and at least one other lamina of thermoplasticmaterial, and wherein volumetrically heating the layered arrangementfurther includes melting the first lamina and the at least one otherlamina of thermoplastic material.
 26. The method according to claim 14,wherein providing at least two laminae includes providing a first laminaof thermoset material and at least one other lamina of thermosetmaterial, and wherein volumetrically heating the layered arrangementfurther includes effecting partial curing of the layered arrangement.27. The method according to claim 14, wherein the volumetrically heatingis carried out on a first portion of the layered arrangement, thesubstantially simultaneously cooling and applying pressure is carriedout on a second portion of the layered arrangement and the quenching ina directionally controlled manner is carried out on a third portion ofthe layered arrangement in a substantially simultaneous manner.
 28. Amethod of forming a composite laminate structure, the method comprising:providing at least two laminae containing electrically conductivereinforcing fibers, the at least two laminae being configured in alayered arrangement; surface heating the layered arrangement prior tovolumetrically heating the layered arrangement; volumetrically heatingthe layered arrangement by inductively transferring energy to theelectrically conductive reinforcing fibers of the at least two laminae;substantially simultaneously cooling and applying pressure to at least aportion of a surface of the layered arrangement; and quenching thelayered arrangement in a directionally controlled manner with respect toa midplane thereof, wherein the surface heating, the volumetricallyheating, the substantially simultaneously cooling and applying pressure,and the quenching in a directionally controlled manner are carried outas a continuous process.
 29. A method of forming a composite laminatestructure, the method comprising: providing at least two laminaecontaining electrically conductive reinforcing fibers includingconfiguring the at least two laminae in a layered arrangement andorienting the reinforcing fibers of one lamina of the at least twolaminae at a specified angle relative to the reinforcing fibers of atleast one other lamina of the at least two laminae; surface heating thelayered arrangement prior to volumetrically heating the layeredarrangement volumetrically heating the layered arrangement byinductively transferring energy to the electrically conductivereinforcing fibers of the at least two laminae; substantiallysimultaneously cooling and applying pressure to at least a portion of asurface of the layered arrangement; and quenching the layeredarrangement in a directionally controlled manner with respect to amidplane thereof.
 30. A method of forming a composite laminatestructure, the method comprising: providing at least two laminaecontaining electrically conductive reinforcing fibers, the at least twolaminae being configured in a layered arrangement; volumetricallyheating the layered arrangement by inductively transferring energy tothe electrically conductive reinforcing fibers of the at least twolaminae; substantially simultaneously cooling and applying pressure toat least a portion of a surface of the layered arrangement; andquenching the layered arrangement in a directionally controlled mannerwith respect to a midplane thereof, wherein the volumetrically heatingis carried out on a first portion of the layered arrangement, thesubstantially simultaneously cooling and applying pressure is carriedout on a second portion of the layered arrangement and the quenching ina directionally controlled manner is carried out on a third portion ofthe layered arrangement in a substantially simultaneous manner.